From all my readings, and experience, this is the problem with all small speakers. Ok, maybe there are a few small speakers that have good bass (PSB twenty22?), but they are so expensive you might as well buy a tower. So, wouldn't a sub (or two) resolve this problem, and also add some other benefits (like offloading the imagine B's so their sound would be even better, etc..)?

The question I've been pondering this weekend is this: given a set price to spend on speakers (say $2000), is it best to buy two towers/floorstanders ($1000/each), or, two bookshelf speakers ($750 each) and a $500 sub?